Historically, hemp fibers have been used in the textile industry. However, recent breakthroughs in materials science have allowed strong and renewable fibers, for example those from hemp, to replace glass fibers as reinforcement in composite materials. Development of protocols to extract hemp fibers while maintaining their integrity is an important aspect to their use in both the textile industry and in composite materials. Such protocols preferably avoid the use of hazardous and/or non-biodegradable agents.
In common fiber plants, e.g. hemp, flax and jute, a bark-like layer containing bast fibers surrounds a woody core. The bast fibers are surrounded by pectin or other gums. Decortication, either manually or mechanically, is a Process to remove the bark-like layer from the woody core.
Extraction of fiber from the decorticated bark would allow its eventual usage. Extraction primarily involves removal of pectin and colour-containing materials from the fiber (degumming). Pectin is a polysaccharide which is a polymer of galacturonic acid. Pectin is not soluble in water or acid. However, it can be removed by strong alkaline solutions like caustic soda (concentrated sodium hydroxide).
General methods for isolation of clean fibers include dew retting, water retting, and chemical and enzymatic processes, with different variations. In these methods, the glue that holds the fibers together must first be loosened (or removed altogether) by retting. In conventional retting, stalks are dew-retted by allowing them to lie in the field after cutting. In some areas of the world, hemp is water-retted by placing bundles of stalks in ponds or streams. These retting approaches depend on digestion of pectin by enzymes secreted by microbes thriving under favorable conditions. Although water-retting yields more uniform fiber, the process pollutes the water. Dew-retting requires anywhere from two to six weeks or more to complete, requiring the stalks to be turned at least once for highest-quality fiber. Dew-retting is thus affected by the weather, which offers no guaranty of favorable conditions.
On an industrial scale, chemical retting is common. It involves violent, hazardous chemicals like soda ash, caustic soda and oxalic acid. Enzyme retting involves the action of pectinase with or without other enzymes like xylanase and/or cellulase. However, the practical application of such enzymes for isolation of hemp fiber remains to be proven.
Various retting processes are known in the art. Clarke et al (2002) describes a process of removing pectin or gummy materials from decorticated bast skin to yield individual fibers by placement of the bast skin (with or without soaking in an enzyme solution in a pretreatment process) into a closed gas-impermeable container such as plastic bag. The enzyme-producing microbes natural to the bast skin, will thrive on the initial nutrients released by the enzyme pretreatment and will finish the retting process in this closed environment. Clarke et al (2002) also describes an alternative pre-treatment process involving chemicals instead of enzymes, and this includes caustic soda, soda ash, sodium silicate, oxalic acid and ethylenediaminetetraacetic acid (EDTA).
Both chemical and enzyme retting processes generally utilize common chelating agents like oxalic acid and EDTA to expedite the process. All have their problems in application and disposal on an industrial scale. Oxalic acid is classified or designated as a toxic, corrosive and hazardous material (particularly to the kidneys) by various jurisdictions (WorkSafe criteria). EDTA is very inert with no or negligible ability to biodegrade in the environment. EDTA is found in many natural waters and occurs at higher levels in wastewater effluents. EDTA has already been banned in Western European countries, in Australia and in parts of the United States of America, and many countries severely restrict or carefully control EDTA as a component in detergents or washing agents.
Thus, there is a need for a milder process for isolating hemp fibers that involves environmentally-friendly and/or biodegradable agents.